The 3rd Generation Partnership Project (3GPP) proposes a next generation network architecture that is based on the Internet Protocol (IP), that is, an IP multimedia subsystem (IMS). Compared with a traditional circuit switched (Circuit Switch, CS) domain, an IMS domain can provide abundant multimedia services for a user.
3GPP further proposes an IMS centralized services (ICS) network architecture. In the ICS network architecture, the traditional CS domain is degraded to an access network of the IMS domain. As a core network, the IMS domain controls all user services, so that consistency and continuity of service experience on a network such as a 2G network, a 3G network, or Long Term Evolution (LTE) can be achieved. Currently, an implementation manner of ICS is upgrading a mobile switching center (MSC) on a live network to a mobile access gateway control function (mAGCF) device, so that an MSC domain supports an ICS function, thereby implementing interworking between the CS domain and the IMS domain.
Upgrading the MSC on the live network is a gradual process. When the MSC is not upgraded, an anchoring manner needs to be used to implement that all supplementary services in the CS domain are controlled by the IMS domain. For example, if user equipment (UE) is located in a CS domain, when the UE initiates a call as calling user equipment or receives a call as called user equipment, the call is routed to an IMS domain on a home network of the UE, and the IMS domain provides a service.
After the UE roams to a visited network, for example, after the UE roams from a home country to a foreign country, if all services are still controlled and provided by the IMS domain on the home network of the UE, the visited network of the UE needs to return, to the IMS domain on the home network in real time, information about a service that needs to be triggered, so that the IMS domain on the home network triggers the corresponding service according to the returned information. However, some differences exist between a CS domain on the visited network and the IMS domain on the home network, for example, timers are inconsistent duration, or signaling mechanisms are different. In this case, when the UE is used as the called UE, many services that need to be triggered cannot be normally triggered. For example, for a late forwarding service, it is possible that a failure cause value cannot be returned to the IMS domain on the home network, and the IMS domain on the home network cannot trigger the late forwarding service according to the failure cause value.